Drum Level Control
Drum Level Control
Drum Level Control
1
Proper boiler operation requires that the level of
water in the steam drum should be maintained within
certain band. A decrease in this level may uncover boiler
tubes, allowing them to become overheated. An increase in
the level of water may interfere with the internal operation
of internal devices in the boiler drum. It is important to
made that the water level in the boiler drum must be
above 50% all the time.
The water level in the boiler drum is related to, but
is not a direct indicator of , the quantity of water in the
drum. At each boiler load, there is different volume in the
water that is occupied by steam bubbles. So if load is
increased there are more steam bubbles and this cause
water to ‘swell’ or rise, rather than fall because of added
water usage.
2
Partial vaporisation in evaporating tubes
3
(1) Single element control system
For small boilers having relatively high storage volumes
and slow changing loads ,single element control system
is used. It controls feed water flow based on drum level.
Response is very slow because a change in feedwater
flow takes a long time to show up the level change. As a
result the steam drum causes water to increase and
decrease in volume, resulting in false measurements.
4
Single element control
5
(2) Two element control system
6
Two element control
7
(3) Three element control
8
Three element control
THREE ELEMENT CONTROL
STEAM
X FT 2
LT3
DRUM Computing
X Level Relay
Controller
Feedwater
FC
Flow
Controller
X FT
1
FV Feedwater
9
10
(4) Five element control
11
Five element control
STEAM
TT X FT
5 2
Temperature
X compensated
steam flow
PT f(x)
4
Pressure
Compensated
Drum Level
DRUM Computing
Level Relay
Controller
FC
Flow
Controller
FT
X
1
FV Feedwater
12
(5) Seven element control
Transmitters for blow down flow and sootblower flow
could be added to five element control to make up
seven element feedwater control.
13
Seven element controll
STEAM
TT X
FT
5 2
Temperature
X compensated
steam flow
PT f(x)
4
Pressure
Compensated
LT3 Drum Level
DRUM Computing
X X Level Relay
Controller
B o ile r T u b e
FT f(x)
6 FC
Drum Blowdown
Flow
Controller
FT f(x)
7
Tubes Blowdown FT
X
Economiser 1
FV Feedwater
14
Advantages
15
Disadvantages
16